1. Field of the Invention
This invention relates generally to sprag type locking devices which operate between a drive member and a reaction member and more particularly to a roll-unlocking roller type 3-D sprag device.
2. Description of the Prior Art
Roller type locking brakes including sprag members are well known and have been used extensively for many years because of their unrivaled torque-holding capability while requiring a relatively small package. Also they find favor because they are low power devices and never overheat, because they remain locked with the power off, and because they have extremely long wear life. Such a brake is unique in its ability to release and reengage under full load and does so using rolling friction. Moreover, it excels in applications where safety is paramount, such as overhead cranes, material handling equipment, machine tools and robots.
More recently, the three-dimensional (3-D) roller locking sprag has been developed. Such a structure is shown and disclosed in the above cross-referenced application, U.S. Ser. No. 08/280,979 (GSC13,617), now U.S. Pat. No. 5,482,144. Such apparatus provides an improvement in sprag-type devices which effectively doubles the sprag locking force without increasing the associated contact stresses.
Also, in the above cross-referenced application, U.S. Ser. No. 08/598,298 (GSC13,674), there is shown and described a screw-released roller brake assembly including an input drive assembly, an output drive assembly, a plurality of 3-D locking sprags, a mechanical tripper nut for unlocking the sprags, and a casing therefor. The sprags operate to lock the output drive assembly to the casing so as to prevent rotation thereof in an idle mode of operation. In a drive mode of operation, the tripper is either self-actuated or motor driven so that it is translated linearly up and down against the spline, whereupon at the limit of its travel, the sprags are caused to rotate and unlock while coupling the input drive assembly to the output drive assembly so as to impart a turning motion thereto in either a clockwise or counterclockwise direction.
Sprag and roller and ball locking devices characteristically depend on each rolling member trying to roll over its center and, in the process, creating severe frictional forces as it locks up. The generation of these large frictional forces makes it extremely difficult to free these systems once they are under load. It also means that large radial forces are generated which stress the structure and the rolling members. This stress is relieved by a factor of two or more in the case of 3-D sprags, but is not eliminated. One still cannot disengage and reengage under load using rolling friction. The friction must essentially be overpowered or relieved before the disengagement. If the friction is overpowered, wear will likely result and with it, unwanted particles are produced which can become harmful when located, for example, in a piece of equipment such as a telescope in a spacecraft.